1. Field of the Invention
The present invention relates to methods and related systems for producing a representation of acoustical impedances of materials and, more particularly, to such methods and systems that use ultrasonic energy for medical diagnostic purposes.
2. Setting of the Invention
The use of ultrasonic energy to obtain meaningful information about the internal structure and condition of living organisms has been a great advance over prior invasive techniques. Over the past number of years the use of ultrasonic energy for such medical diagnostic purposes has greatly increased, yet the information being presented to the operator, usually in the form of a visual image, is still very difficult to interpret. A skilled operator is needed for the interpretation of the image. There is a great need for a method and system that can improve the quality and detail of the image so that better medical diagnoses can be obtained.
Conventional ultrasonic imaging devices fall into a number of categories. Some of these are used to study systems containing boundaries where the acoustic impedance, Z, of a material (Z.ident..rho.v, where .rho. is the density and v is the velocity of sound in the material) differs greatly from one material to the next. In such a system, the reflections are distinct and multiple reflections are common. In systems such as living tissue the difference of impedance from one material to the next (such as from muscle to fat) make for reflections that are difficult to detect and differentiate. Research is being conducted to find ways to improve the signal processing of these signals, but also into ways to improve the signal that is sent into the body.
Ultrasonic imaging usually uses a piezoelectric crystal that is placed against the body to be imaged. The crystal is subjected to a single voltage spike (.about.200 volts for a duration of tens of nanoseconds). The crystal deforms along the electric field and because of the contraction and dilation of the crystal, it produces sound at one end of the crystal. The other end of the crystal is held firmly by a sound absorbing material, such as a plastic with heavy metal particles imbedded in the plastic.
Although the impressed voltage is well defined, the crystal is not particularly cooperative in tracking the applied voltage. What is seen are multiple oscillations which usually dampen out in four or five cycles at a frequency determined by the physical characteristics of that particular crystal.
Conventional imaging devices either view the raw ultrasonic signal received from the body or employ techniques based on the fourier transform to yield an image. One fourier technique uses the calculation of a correlation function which tries to decompose the received signal into a sequence of the transmitted signal. Phase cancellation of one reflection with another make this signal extremely difficult to analyze by fourier techniques. A map of the raw signal of a perfectly sharp boundary produces an imprecise blur at least to the extent that the transmitted signal is smeared in time. There is a need for a method and related system for transmitting a signal from a crystal that can be more precisely created, in order to be more precisely detected and imaged.